1. Field of the Invention
The present invention relates to a fuel cell system in which fuel gas and oxidant gas are supplied to a fuel cell to generate electricity and a shutdown method of the fuel cell system.
2. Description of the Related Art
A fuel cell system is a power generation system in which fuel gas containing hydrogen and oxidant gas containing oxygen are supplied to a fuel cell and are electrochemically reacted with each other for extraction of electrical power. Such a fuel cell system is used as power sources of fuel cell vehicles, for example.
With regard to this type of fuel cell system, the following matters are known. When fuel and oxidant electrodes of the fuel cell are filled with air because of ingress of air during a system leaving period while the operation of the fuel cell system is stopped (hereinafter, referred to as a system stop period), a boundary part of a gas composition called a hydrogen front is formed at the next start-up of the fuel cell system by air remaining at the fuel electrode and fuel gas newly supplied. The presence of the hydrogen front causes corrosion of a carbon support supporting a catalyst of platinum or the like, thus degrading the performance of the fuel cell. Accordingly, various start-up methods to prevent such degradation of the performance of the fuel cell have been studied.
Such degradation of the performance of the fuel cell is caused because the fuel and oxidant electrodes of the fuel cell are filled with air during the system stop period after the operation of the system stops. Accordingly, it is effective to address optimization of the condition of the fuel cell at stopping the operation the fuel cell system to avoid the aforementioned problem occurring at the start up of thereof.
Japanese Patent Unexamined Publication No. 2006-73376 describes a technique related to a system shutdown process for optimization of the condition of the fuel cell during the system stop period. The system shutdown process described in the above publication is as follows. At stopping the operation of the system, first, the fuel cell is disconnected from an external load and then connected to an internal discharge load. Supply of the oxidant gas to the oxidant electrode of the fuel cell is then stopped, and open-close valves provided at inlet and outlet sides of the oxidant electrode are closed. Current from the fuel cell is applied to the internal discharge load to consume oxygen at the oxidant electrode. The pressure at the fuel electrode of the fuel cell is then increased to bring the fuel and oxidant electrodes in a fuel gas atmosphere.